Investigators believe the suspect pulled the school's fire alarm to draw people out and get a higher death toll, according to a law enforcement source.

There had been a fire drill at the school earlier in the day, leading some to believe at first that the afternoon incident was another drill, a student told CNN affiliate WSVN. "Everyone just started freaking out."

"But then word started going around that it was shots and not just, like, something else, everyone just started running towards the canal," the student said.